Numerous applications exist where monitoring of a fluid level is important. For example, one application is in monitoring the level in an LPG tank, where both a visual indication of fluid level and electrical indication are desirable.
In the early sixties, Rochester Gauges, Inc. of Dallas, Texas introduced a magnetically driven variable resistor known as a "twin site" sender. This design utilized an annular resistance wire and cooperating arm in the electric circuit, and a pointer and dial for a visual output of fluid level.
The design of this sender included a formed metal base, on which is formed the dial face. The annular resistance wire was connected at one end to an external contact. A magnetically driven pointer assembly, having a resilient contact to sweep along the resistance wire, was grounded through the case by contact between the pointer assembly and the pointer stem on which the pointer assembly pivoted.
The magnetic drive feature is important because some fluid products, such as LPG, are stored under pressure. The magnetic drive allows the signal from the float mechanism inside of the tank to be transmitted through a solid, non-magnetic bulkhead without the necessity of dynamic seals or pressure tight conductors.
While a significant advance in the art, and still used today, a need exists for an improved sender which can be manufactured for less cost and be as reliable or more reliable than the original sender. Further, occasionally the ground contact between the pointer assembly and the case would be affected by corrosion, leading to a false resistance read out.